As new generations of handsets, gateways, and other wireless communication devices become embedded with more applications, and the need for increased bandwidth becomes greater, new antenna systems are required to optimise link quality. Specifically, better control of the radiated field is required to provide better communication link quality with intended transceivers, whilst suppressing signals from undesired transceivers. Typically, during normal operation of a wireless communication device, impedance and/or frequency changes at the antenna create mismatches. It is known that such impedance mismatches and/or frequency resonance mismatches have a significant impact on the power transfer through the antenna system. For example, such mismatches may occur if the antenna is moved and repositioned near a reflective or (radio frequency) absorbent material, or if the antenna is positioned closer or further away from, say, a head of a user. Maximising energy transfer through antenna systems is a key desirable performance criteria and intelligent and adaptable antenna systems to address such mismatches are needed.
Antennas are transition devices (e.g. a form of transducer) that function between guided waves and free space waves. There are two primary forms of tuning in antennas: antenna aperture tuning, which adds components at the antenna aperture or ground point to adjust the antenna resonance frequency, and antenna impedance tuning, which is executed by adjusting the impedance matching at the antenna feed point.
Referring to FIG. 1, implementation details of the different techniques of antenna aperture tuning and antenna impedance tuning are illustrated. A first device 100 comprises a tunable antenna 110 with an aperture tuner 109. An aperture tuner 109 optimizes the radiation efficiency of the antenna system. Radio frequency (RF) transceiver and modem circuit 116 comprises a transmit input path 102 for generating signals to be radiated by the antenna 110 and a receiver output path 104 to carry signals that have been received by the antenna and are subsequently processed.
Frequency resonance mismatches are a particular problem, as they can adversely affect an antenna's radiation efficiency and bandwidth. Therefore, in this document, aperture tuner 109 is arranged to perform a limited form of closed loop 108 aperture tuning with measurements made at the aperture tuning connection point 120. Notably, with most known aperture tuning arrangements, open loop measurements are made, as illustrated in FIG. 2.
A second known antenna tuner technique 150 comprises an antenna 160 operably coupled to an impedance tuner 158 via an antenna feed 156. The impedance tuner 158 is operably coupled to a coupler 162 to extract portions of signals passing there through to facilitate impedance measurements. In this arrangement, a controller 174 is operably coupled to coupler 162 and arranged to receive representations of forward power and reverse power to enable the controller to determine impedance measurement information. The controller 174 is operably coupled to impedance tuner 158 via control line 178 and arranged to adjust the tuner to provide an optimal match to the antenna impedance in response to the impedance measurement information. In this manner, the antenna tuner technique 150 is arranged to perform closed loop control of the antenna impedance matching. Antenna impedance tuning attempts to optimize the power transfer between the transmission line feed and the antenna element.
The coupler 162 is operably coupled to RF transceiver and modem circuit 166 via RF path 170. RF transceiver and modem circuit 166 comprises a transmit input path 102 and a receiver output path 104. Impedance mismatches between the antenna 160 and the RF transceiver and modem circuit 166, as well as objects close to the antenna 160 that affect the radiation field, can cause further undesirable losses. Thus, to reduce some of these losses, it is known to utilise coupler 162, controller 174 and impedance tuner 158 in a closed loop impedance matching network, in order to compensate for any impedance mismatches.
Referring to FIG. 2, a known antenna arrangement 200 for a wireless communication unit is illustrated. The antenna arrangement 200 is a Planar Inverted-F Antenna (PIFA) 202, which is a common antenna arrangement used for smartphones. The PIFA 202 comprises a shorting pin 204 that couples the PIFA 202 to a ground plane 206. The PIFA 202 comprises a radiating element 208 arranged to radiate/receive free space waves 210.
In this PIFA 202, aperture tuning is performed by an aperture tuner 203 that couples the PIFA 202 to a ground plane 206. The aperture tuning operation is performed by optimizing the radiation efficiency from the antenna terminals into free space by tuning the antenna resonance frequency. Aperture tuning is performed at a point that is distal from the antenna feed point 214 and is usually performed open loop using look-up tables for the aperture tuning state (i.e. with no feedback information to influence the tuning operation in a real-time manner).
Impedance matching for the PIFA 202 is performed by impedance matching circuit 201, which presents a suitably adjusted impedance at antenna feed point 214. The antenna impedance tuning is arranged to optimize power transfer to/from the transmission line 216, e.g. in a form of a guided wave, from/to the antenna element(s) by tuning the impedance matching circuit 201. A main signal connection is coupled to antenna feed point 214 via impedance matching circuit 201, with a small portion extracted by directional coupler 222, and the resulting forward and reverse (reflected) signals are measured using measurement unit 224. The measurement unit 224 is then able to adjust, via a control signal applied to path 226, one or more variable component(s) in the impedance matching circuit 201 to adjust the impedance match between transmission line 216 and antenna feed point 214.
Referring to FIG. 3, an antenna resonance occurs when the imaginary part of the antenna input impedance is zero (imag{Zin}˜0). To achieve good antenna efficiency it is not sufficient that the antenna is in resonance, indicated by imag{Zin}˜0, but simultaneously the real part of the antenna input impedance should be close to the characteristic impedance of the transmission line connected to the antenna feed point, which is typically 50Ω (real{Zin}˜50Ω). The PIFA antenna in FIG. 3 exhibits multiple antenna resonances (twelve in this example) as indicated by the zero crossings of the imag(Zin) curve in the top plot. For many of these resonances the real part of the antenna input impedance real{Zin} is far from 50Ω, for example for the resonance at 306 where the real (Zin) is close to zero. However, as shown, two resonances, one low-band (LB) 302 and one high band (HB) 304 exhibit a real input impedance close to 50Ω. Note that these two optimum resonances correspond to a condition where the input reflection or S11 at the antenna feed point is low as illustrated in the bottom plot in FIG. 3. Hence, in implementing aperture tuning, such as by aperture tuner 203 of FIG. 2, selecting an optimal resonance for a particular impedance is problematic.
Therefore, known techniques for compensating for antenna resonance frequency detuning and antenna impedance mismatches are typically not sufficient to result in adequate antenna efficiency over a required wide bandwidth. Therefore, there may be a need to provide a different antenna tuner technique.